The present invention relates to differential amplifiers in general, and in particular to a differential amplifier which recovers rapidly from an overdriven state to a linear operating state.
A typical transistor-based differential amplifier produces a differential output signal which varies in a substantially linear fashion with a differential input signal over its normal operating range. However, when the input signal becomes too large in either a positive or negative sense, the amplifier becomes "overdriven" such that base-emitter voltages of transistors in the differential amplifier are driven outside a range for which the transistors are active, and such transistors may saturate or shut off. When a transistor saturates or shuts off, the charge on inherent circuit capacitance at the base of the transistor may increase or decrease substantially, and in order for the transistor to recover from a saturated or shut off state to an active state, its base capacitance must be appropriately charged or discharged. Consequently, to permit a differential amplifier to recover from an overdriven state, the input signal magnitude must be reduced to within its normal range and the capacitances at the bases of saturated or shut off transistors in the amplifier must be charged or discharged by an amount sufficient to permit the transistors to enter their active regions. Since the charging or discharging of these capacitances is time consuming, there is a delay between the time the input signal re-enters the range for which the amplifier normally responds to the input signal in a linear fashion and the time the amplifier actually begins to respond to the input signal in a linear fashion. What is needed is a differential amplifier which recovers to a linear operating state from an overdriven state with minimal delay.